The present invention concerns formation of contact holes conducted as a step in the production of semiconductor devices.
When semiconductor devices of multi level interconnect structures are manufactured, contact holes are formed in dielectric interlayers so as to connect upper and lower wiring layers to each other. A step of forming contact holes has been conducted, for example, as described below.
At first, a silicon oxide film is formed as an insulating film on an underlying aluminum wiring layer. Then, a resist film is formed on the silicon oxide film. A mask pattern for contact holes is transferred to the resist film by a photolithographic technique to form a resist pattern. Then, dry etching is applied to the silicon oxide film through the resist pattern.
As an etching gas for the dry etching, a gas mixture of CF4 and CHF3 is used, for instance. Further, in addition to the etching gas, an Ar gas and an N2 gas are supplied in the chamber during etching with the following purpose. The Ar gas is added for the dilution of the etching gas (CF4+CHF3).
Nitrogen is added mainly for two purposes. One of them is to prevent retrogradation of the resist (scraping of the contact hole edge of the resist pattern) during etching. The other of them is to suppress formation of a polymer, which increases the wiring resistance, on the side wall of the contact holes during etching, and making it readily removable by an organic cleaning liquid or the like even when the polymer is formed.
After thus forming the contact holes, a conductor connecting the upper and the lower wiring layers is deposited into the contact holes (for example, tungsten plugs are formed) and an upper aluminum wiring layer is formed.
However, the semiconductor device of the multi level interconnect structure obtained in the prior art described above involves a problem that the wiring resistance tends to be increased and the wiring resistant is varied in one identical lot.
The present invention has been accomplished taking notice on such a problem in the prior art and it is a subject thereof to make the wiring resistance lower and reduce the variation of the wiring resistance in one identical lot in a semiconductor device of a multi level interconnect structure which at least the under wiring layer comprises an aluminum wiring layer.
For solving the foregoing subject, the present invention provides a method of forming contact holes which comprises forming an insulating film on an aluminum wiring layer and applying dry etching to the insulating film by using a predetermined etching gas through a resist pattern, thereby forming contact holes, wherein the method comprises conducting a first etching step of applying etching with addition of nitrogen to an etching gas for a predetermined period of time and then conducting a second etching step of applying etching with no addition of nitrogen to the etching gas.
In the method of the present invention, it is preferred that the processing time of the first etching step is substantially equal with the time in which the minimum thickness portion of the insulating film is entirely etched in the direction of the thickness.
The etching gas used in the method of the present invention can include a gas containing a fluoro carbon series gas.
The aluminum wiring layer used in the method of the present invention can include an aluminum alloy layer comprising an aluminum alloy containing copper (Cu) and/or silicon (Si).
When etching with addition of nitrogen to the etching gas is applied continuously also after the etching depth has reached the surface of the aluminum wiring layer, AlN is formed to the surface of the aluminum wiring layer as the bottom of a contact hole. On the contrary, it is supposed that such AlN is not formed in the etching conducted with no addition of nitrogen to the etching gas. Then, it is considered that presence of AlN increases the wiring resistance of the resultant semiconductor device of the multi level interconnect structure more in a case of forming the contact hole by the etching method with addition of nitrogen to the etching gas than that in a case of forming the contact hole by the etching method with no addition of nitrogen to the etching gas.
Accordingly, it is preferred that the contact hole is formed by the etching method with no addition of nitrogen to the etching gas in order to lower the wiring resistance. However, this etching method can not provide the effect of preventing the retrogradation of the resist and the effect of suppressing the formation of the polymer to the side wall of the contact hole. As a result, this leads to a problem that the contact hole is formed to a size larger than the predetermined value and the wiring resistance is increased by the effect of the polymer.
On the contrary, in the method according to the present invention, etching with no addition of nitrogen to the etching gas (second etching step) is applied after applying an etching with addition of nitrogen to the etching gas (first etching step) for a predetermined time. Therefore, the effect of preventing the retrogradation of the resist and formation of the polymer described above can be obtained till lapse of a predetermined time from the start of the etching, and the effect of preventing formation of AlN described above can be obtained after lapse of the predetermined time.
Accordingly, if the processing time in the first etching step is properly set depending on the thickness of the insulating film in the upper portion of the aluminum wiring layer, the retrogradation of the resist and the formation of the polymer can be reduced with suppressing the formation of the AlN.
Particularly, if the processing time in the first etching step is made substantially equal with the time in which the minimum thickness portion of the insulating film on the aluminum wiring layer is entirely etched in the direction of the thickness, the first etching step is completed substantially at the time the etching depth for the minimum thickness portion of the insulating film reaches the surface of the aluminum wiring layer. Therefore the formation of AlN can be prevented substantially completely.
As a result, the contact holes can be prevented from being formed larger than the size of the predetermined value, as well as the wiring resistance of the resultant semiconductor device of the multi level interconnect structure can be kept lower and the variation of the wiring resistance in one identical lot can also be reduced.